The present invention relates to a fuel assembly for nuclear reactors, and particularly to the general type comprising a box-like housing with upper and lower grids therein for supporting elongated fuel rods in such a manner as to define coolant flow channels thereamong. A fuel assembly of this kind is disclosed in U.S. Pat. No. 3,697,375, in which fuel rods are supported in grid channels formed with circular-cylindrical cross section and each circular grid channel is surrounded by four adjacent coolant channels.